1. Field of the Invention
The present invention relates to an object lens actuator used in an optical disc drive that writes/reads information in/from an optical disc. Particularly, the invention relates to an object lens actuator in which the stiffness of an object lens holder is improved.
2. Background Art
In an optical disc write/read drive that writes/reads information in/from a disc-type information recording medium, attempts have been made to improve the data transfer speed by spinning the disc at high speed. In order to accurately write/read information in/from the rapidly spinning optical disc, such optical disc write/read drive mounts an object lens actuator that drives an object lens in the focusing direction and the tracking direction so that the lens follows a recording surface of the optical disc.
A general object lens actuator comprises a magnetic circuit composed of a yoke and a permanent magnet, a movable lens holder that holds an object lens, a fixed part that holds the lens holder, and a support member that elastically supports the lens holder with the fixed part. A focusing coil and a tracking coil are attached to the lens holder, and when a current is caused to flow through the focusing coil, the lens holder is caused to be driven in the focusing direction due to electromagnetic force produced by the effect of magnetic flux from the permanent magnet attached to the yoke. Similarly, when a current is caused to flow through the tracking coil, the lens holder is caused to be driven in the tracking direction due to electromagnetic force produced by the effect of magnetic flux from the permanent magnet attached to the yoke.
In such object lens actuator that performs the positioning of an object lens, it is necessary to increase the control band for focusing and tracking as the number of rotations of the optical disc is increased. Further, in order to realize stable control, it is necessary to increase the higher order resonance frequency of the object lens actuator. Since the higher order resonance of the object lens actuator is due to the elastic vibration of the lens holder, in order to increase the higher order resonance frequency, it is necessary to improve the stiffness of the lens holder.
Patent Document 1 discloses a method for improving the stiffness of such lens holder by coupling a lens holding part and a coil placing part with each other with thin-wall reinforcement ribs and plugging at least a part of the opening parts on the side of the upper end face of the space surrounded by the outside part of the lens holding part, the coil placing part, and the reinforcement ribs, with a thin plate. In the object lens holder disclosed in Patent Document 1, it is considered that a lighter lens holder can be obtained and the higher order resonance frequency of the lens holder can be increased by making the reinforcement ribs thin-walled and by making the plate for plugging the opening parts on the side of the upper end face thin.
Patent Document 1: JP Patent Publication (Kokai) No. 9-198682 A (1997) (claim 1, FIG. 1, and the like)